1. Field of the Invention
The present invention relates to a perpendicular recording medium and a magnetic recording apparatus incorporating the perpendicular recording medium.
2. Description of the Related Art
A magnetic thin film formed mainly of Co has a high saturation flux density and a prominent magnetocrystalline anisotropy, thus the film is now used for magnetic recording media more and more widely. In particular, by utilizing a great anisotropy energy that the magnetic thin film has, attempts are made to use the film in manufacturing perpendicular recording media suitable for ultra-high density recording. The results of research conducted in recent years reveal that, in order to improve coercivity and media S/N ratio, Co-based perpendicular magnetization film needs to be formed into such a structure that magnetic interactions between Co-based grains are separated.
Further, it is known that a double-layer medium, which comprises a soft magnetic underlayer having a high permeability and a perpendicular magnetization film, exhibits better recording-producing characteristics, due to an interaction between a head and the soft magnetic underlayer, than a single-layer medium, which comprises a perpendicular magnetization film only (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 52-78403). Thus, most perpendicular recording media have a soft magnetic underlayer (back layer) under a perpendicular magnetization film.
Various Co-based perpendicular magnetization films are known. Among them are: (1) CoCr alloy film or CoCr alloy film containing an element selected from Ni, Ta and Pt, which are most popular; (2) Co-CoO film formed by vacuum deposition performed in an oxygen atmosphere (See, for example, Jpn. Pat. Appln. KOKAI Publication No. 59-162622); (3) CoPt(Cr) alloy film formed by sputtering (Proceedings of Japan Applied Magnetics Society, 8pB-11, 1990); (4) CoPtBO film (Jpn. Pat. Appln. KOKAI Publication No. 3-58316); (5) multilayer film having a Co layer and a Pt layer, developed mainly for optomagnetic recording (Jpn. Pat. Appln. KOKAI Publication No. 3-80421).
The perpendicular magnetization films (1) to (5) will be described in more detail.
(1) CoCr Alloy Film
A film made of Co only has a shape anisotropy energy due to the thin film shape greater than a magnetocrystalline anisotropy energy. On the other hand, it is expected in a CoCr film in which Cr is added to Co that the shape anisotropy can be reduced and the coercivity can be increased to some degree because of segregation of Cr at the boundaries of Co grains and accordingly suppression of the magnetic interaction between Co grains. Since Cr is mixed in the Co grains, however, the magnetocrystalline anisotropy and saturation magnetization of the CoCr film are less than those of a film made of Co only. It is therefore necessary to use more Cr in order to manufacture a perpendicular magnetization film whose decrease in the shape anisotropy is more than that in the magnetocrystalline anisotropy. A CoCr film having such a high Cr content has but a saturation magnetization of about four times weaker than that of a film made of Co only.
Unlike a longitudinal recording medium, a perpendicular recording medium cannot have an increased surface magnetic flux density merely by increasing the thickness of the recording film. The perpendicular medium therefore needs to have a film with sufficient saturation magnetization to generate a large output. If a CoCr perpendicular magnetization film is formed, the medium would fail to generate a sufficient output because the CoCr film has but a low saturation magnetization as described above.
(2) Co-CoO Film
In a Co-CoO film, the Co grain density is reduced, and the magnetic interaction between the Co grains is weakened since antiferromagnetic CoO having a Neel point close to room temperature is formed at the boundaries of the Co grains. The film can therefore have a relatively large coercivity. If made thicker, the Co-CoO film will likely acquire shape anisotropy energy due to columnar grains, Furthermore, the Co-CoO film is superior to a CoCr film in terms of saturation magnetization.
Since oxygen is strongly bonded with Co, however, (002) oriented Co grains are hard to grow. Consequently, the Co-CoO film exhibits but poor crystalinity and a very low magnetocrystalline anisotropy, its crystal orientation is liable to deteriorate, and it cannot have a high coercivity.
Further, because a remarkable reduction in the magnetocrystalline anisotropy and difference in magnetocrystalline anisotropy between grains arise simultaneously, the Co-CoO film involves a prominent anisotropy dispersion. In particular, CoO has a Neel point in the vicinity of room temperature, and consequently, a bias magnetic field will be applied to the Co grains when the ambient temperature of the medium falls below room temperature. The bias magnetic field becomes locally non-uniform since the orientation of the CoO grains is not uniform. As a result, magnetic dispersion in the Co-CoO film is increased.
Moreover, since the orientation of the Co grains is deteriorated, i.e., the c-axis of Co can hardly be oriented perpendicularly to the substrate, the perpendicular anisotropy energy cannot be greater than the longitudinal anisotropy energy. The Co-CoO film cannot have properties required as a perpendicular magnetization film.
(3) CoPt(Cr) Alloy Film
A CoPt(Cr) alloy film has a high saturation magnetization. Moreover, if formed by sputtering conducted under optimal conditions, the CoPt(Cr) alloy film may have sufficient perpendicular anisotropy energy. Unlike Co-based films containing elements other than Pt, the CoPt(Cr) alloy film containing Pt up to 30 at % has greater perpendicular anisotropy energy than a film made of Co only.
The CoPt(Cr) alloy film, however, has strong magnetic interaction between the grains, and has a low perpendicular coercivity. As a result, a recording medium having this film cannot generate a sufficient output at a low band.
(4) CoPtBO Film
A CoPtBO film can be obtained by adding boron to CoPt and by forming into film in an oxygen atmosphere. This film has fairly high saturation magnetization.
The CoPtBO film, however, is liable to take fcc phase. A perpendicular anisotropy of the film is induced based on a shape anisotropy. The film should therefore be made thick. Since the CoPtBO film tends to be oxidized due to the addition of boron, its crystal orientation is liable to deteriorate as is the case of a Co-CoO film. The CoPtBO film needs to be supported by an underlayer made of, for example, Pt to preserve the crystal orientation.
A perpendicular recording medium generally has a soft magnetic underlayer under the perpendicular magnetization film as described above. If the perpendicular magnetization film is made thick or a Pt underlayer is used to control the crystal orientation, the spacing between a magnetic head and the soft magnetic underlayer will unavoidably increase, diminishing the interaction between the grains.
(5) Multilayer film having Co layer and Pt layer
A multilayer film of this type has an anisotropy energy, which can be attributed to the interface effect between the Co layer and the Pt layer. Hence, the condition of the interface between the Co layer and the Pt layer must be controlled. In other words, anything that would degrade the interface condition should not be done. Thus it is difficult to, for example, increase the deposition rate of the film. Since this film has a multilayer structure, its overall saturation magnetization is comparatively low, or as low as that of a CoCr film.
There is another problem in connection with requirement to minimize the head-to-medium spacing in order to enhance the recording density. The medium has a protective film, made of carbon or the like having a thickness of about 20 nm, for protecting the recording film from damage when a magnetic head collides with the medium. To this end, when the flying height of the head is decreased to a value less than surface roughness Rmax of the medium, the head will contact the medium more frequently. It will then become necessary to make the protective film thicker, making it impossible to reduce the head-to-medium spacing.
When recording-reproducing is performed with a double-layer perpendicular recording medium described above, there will be detected spike noise. The spike noise is not detected in a single-layer recording medium having a perpendicular magnetization film only. This noise is generated not from the interaction between the soft magnetic underlayer and the perpendicular magnetization film formed on the underlayer. Rather, it is generated exclusively in the soft magnetic underlayer. This noise is generated not uniformly in the entire medium: That is, many domain walls exist in the soft magnetic underlayer where the spike noise is generated, and no domain walls exist where no spike noise is generated (Jpn. Pat. Appln. KOKOKU Publication No. 3-53686). The spike noise is generated due to the irreversible movement of the domain walls, and is generally called "Barkhausen noise". To suppress this noise, it suffices to inhibit the generation of domain walls in the soft magnetic underlayer. However, no effective means for suppressing this problem is available.
Furthermore, in order to obtain more excellent recording-reproducing characteristics, it is important to develop a magnetic recording apparatus wherein a perpendicular recording medium and recording and reproducing heads are combined in optimal manner.